HRTBs with two lifetimes: implementation is not general enough when impl uses lifetime bound

[JanBeh]

I tried this code:

fn foo<'a, 'b: 'a>(_arg: &'a &'b i32) {}

trait Trait<T> {
    fn method(&self, arg: T);
}

impl<'a, 'b> Trait<&'a &'b i32> for ()
where
    'b: 'a, // `main` compiles if this line is commented out
{
    fn method(&self, arg: &'a &'b i32) {
        foo(arg);
    }
}

fn works_fine1<'a, 'b>(arg: &'a &'b i32) { foo(arg) }
fn works_fine2<T: for<'a, 'b> FnMut(&'a &'b i32)>(_arg: T) {}
fn fails<T: for<'a, 'b> Trait<&'a &'b i32>>(_arg: T) {}

fn main() {
    works_fine1(&&0);
    works_fine2(|x| foo(x));
    fails(());
}

(Playground)

I expected the program to compile and run without errors because 'b: 'a irregardless of explicitly adding that bound (as shown by calling foo).

Instead, this compilation error occurred:

error: implementation of `Trait` is not general enough
  --> src/main.rs:23:5
   |
23 |     fails(());
   |     ^^^^^^^^^ implementation of `Trait` is not general enough
   |
   = note: `()` must implement `Trait<&'0 &'1 i32>`, for any two lifetimes `'0` and `'1`...
   = note: ...but it actually implements `Trait<&&'2 i32>`, for some specific lifetime `'2`

Minimal example:

trait Trait<T> {}
impl<'a, 'b: 'a> Trait<&'a &'b i32> for () {}

fn fails<T: for<'a, 'b> Trait<&'a &'b i32>>(_arg: T) {}

fn main() {
    fails(());
}

(Playground)

The minimal example will compile if we replace:

-impl<'a, 'b: 'a> Trait<&'a &'b i32> for () {}
+impl<'a, 'b> Trait<&'a &'b i32> for () {}

Meta

rustc --version --verbose:

rustc 1.73.0-nightly (0308df23e 2023-07-21)
binary: rustc
commit-hash: 0308df23e621e783e31a27ca5beaa01b9df60d4a
commit-date: 2023-07-21
host: x86_64-unknown-freebsd
release: 1.73.0-nightly
LLVM version: 16.0.5

See also

See also: Why can’t I use lifetime bounds in HRTBs? on URLO.

I ran into that problem in a PR #1048 for regex where it wasn't trivial to work around. See also comment below in that matter.

[JanBeh]

Note that it's not always possible to omit the explicit lifetime bound, as shown in the following example:

// we can't omit the `'b: 'a` bound here:
trait OtherTrait<'a, 'b: 'a>: FnMut(&'a &'b i32) {}

impl<'a, 'b, F> OtherTrait<'a, 'b> for F
where
    'b: 'a, // and we can't comment this out
    F: ?Sized + FnMut(&'a &'b i32)
{
}

fn other_func<T: for<'a, 'b> OtherTrait<'a, 'b>>(_arg: T) {}

fn main() {
    other_func(|_x: &&i32| ());
}

Fails with:

error: implementation of `OtherTrait` is not general enough
  --> src/main.rs:14:5
   |
14 |     other_func(|_x: &&i32| ());
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `OtherTrait` is not general enough
   |
   = note: `[closure@src/main.rs:14:16: 14:27]` must implement `OtherTrait<'0, '1>`, for any two lifetimes `'0` and `'1`...
   = note: ...but it actually implements `OtherTrait<'_, '2>`, for some specific lifetime `'2`

(Playground)

If we try to comment out this:

-    'b: 'a, // and we can't comment this out
+    //'b: 'a, // and we can't comment this out

We get:

error[E0478]: lifetime bound not satisfied
 --> src/main.rs:4:17
  |
4 | impl<'a, 'b, F> OtherTrait<'a, 'b> for F
  |                 ^^^^^^^^^^^^^^^^^^
  |
note: lifetime parameter instantiated with the lifetime `'b` as defined here
 --> src/main.rs:4:10
  |
4 | impl<'a, 'b, F> OtherTrait<'a, 'b> for F
  |          ^^
note: but lifetime parameter must outlive the lifetime `'a` as defined here
 --> src/main.rs:4:6
  |
4 | impl<'a, 'b, F> OtherTrait<'a, 'b> for F
  |      ^^

And if we try to fix that:

-trait OtherTrait<'a, 'b: 'a>: FnMut(&'a &'b i32) {}
+trait OtherTrait<'a, 'b>: FnMut(&'a &'b i32) {}

Then we get:

error[E0491]: in type `&'a &'b i32`, reference has a longer lifetime than the data it references
 --> src/main.rs:2:27
  |
2 | trait OtherTrait<'a, 'b>: FnMut(&'a &'b i32) {}
  |                           ^^^^^^^^^^^^^^^^^^
  |
note: the pointer is valid for the lifetime `'a` as defined here
 --> src/main.rs:2:18
  |
2 | trait OtherTrait<'a, 'b>: FnMut(&'a &'b i32) {}
  |                  ^^
note: but the referenced data is only valid for the lifetime `'b` as defined here
 --> src/main.rs:2:22
  |
2 | trait OtherTrait<'a, 'b>: FnMut(&'a &'b i32) {}
  |                      ^^

error[E0491]: in type `&'a &'b i32`, reference has a longer lifetime than the data it references
 --> src/main.rs:7:17
  |
7 |     F: ?Sized + FnMut(&'a &'b i32)
  |                 ^^^^^^^^^^^^^^^^^^
  |
note: the pointer is valid for the lifetime `'a` as defined here
 --> src/main.rs:4:6
  |
4 | impl<'a, 'b, F> OtherTrait<'a, 'b> for F
  |      ^^
note: but the referenced data is only valid for the lifetime `'b` as defined here
 --> src/main.rs:4:10
  |
4 | impl<'a, 'b, F> OtherTrait<'a, 'b> for F
  |          ^^

(Playground)

But this compiles:

fn direct<T: for<'a, 'b> FnMut(&'a &'b i32)>(_arg: T) {}

fn main() {
    direct(|_x: &&i32| ());
}

(Playground)

The examples in this comment have also been run using rustc 1.73.0-nightly (0308df23e 2023-07-21).